The Effectiveness of Vaccination with Two Live Infectious Bronchitis Vaccine Strains against QX-Like Infectious Bronchitis Virus Isolated in Thailand

The purpose of this study was to evaluate the level of protection generated by vaccination with 2 live attenuated vaccine strains against QX-like infectious bronchitis virus (IBV) isolated in Thailand. Eighty-four 1-day-old broiler chickens were randomly allocated into 6 groups with 14 chickens in each group. Groups 1-4 were vaccinated with different vaccination programs against IBV. Groups 5 and 6 did not receive any IBV vaccine and served as positive and negative control groups, respectively. At 1 day old, the chickens in groups 1 and 4 were vaccinated with H120 vaccine whereas the chickens in groups 2 and 3 were vaccinated with 4/91 vaccine. At 14 days old, revaccination was done in groups 1 and 2 with H120 vaccine whereas groups 3 and 4 were vaccinated with 4/91 vaccine. At 28 days old, the chickens in groups 1-5 were individually challenged with 104.17 EID50 of QX-like IBV (isolate THA80151). The protection level was evaluated at 7 days post-inoculation. Results showed that the body weight of the vaccinated chickens was higher than the infected but non-vaccinated chickens (p<0.05). Furthermore, the morbidity rate and tracheal histopathological lesion score of the vaccinated chickens were lower than that of the infected chickens that were not vaccinated (p<0.05), although the infection rate of the tracheas was similar. These results suggested that the two live attenuated vaccines administered at an interval of 2 weeks used in this study could induce clinical protection and decrease tracheal histopathological lesions, but could not protect against the infection of a challenge strain.